Considering a typical Global System for Mobile communications (GSM) call connection scenario involving a mobile-to-mobile call, speech data is efficiently encoded at each of the mobile terminals, for example using Adaptive Multi-Rate (AMR) or Enhanced Full-Rate coding which compresses voice data to a rate of, for example, 12.2 k/bits/s, for transmission over the radio interface to respective base stations. Certain equipment located within the network infrastructure, referred to as in-path equipment (IPE) may need to modify or “overwrite” the user speech data with some network generated data, e.g. a short voice announcement or tone. However, this equipment operates on a different coding scheme, typically Pulse Code Modulation (PCM) which compresses voice data at the higher rate of 64 kbits/s.
Provision must be made at the Base Station Controllers (BSC) or other nodes within the network infrastructure such as Media Gateways, for transcoding the compressed voice data between the radio interface compression standard and the network compression standard. Thus voice data is transcoded twice, once from the radio interface compression standard to the network compression standard, the “forward” direction, and once in the reverse direction. Each transcoding step impacts on the quality of the voice signal.
The transcoding entities implemented at or associated with the BSCs are referred to as transcoding units or “TRAUs”. A TRAU may be implemented as a separate entity or together with a BSC. Transcoders (referred to as “TCs”) may exist at other locations within the network, for example at core network boundaries. In some call scenarios, multiple transcoders may be located within the call path.
In order to address the problem of degradation of speech quality due to multiple transcoding stages, a scheme known as Tandem-Free Operation (TFO) has been introduced to GSM and to other mobile communication standards including CDMA and WCDMA based standards. TFO uses the principle that a number of least significant bits of PCM speech samples of the network coded data can be “stolen” to provide an in-band channel for transporting the radio interface coded (AMR or EFR) data. Stolen bits (one every 16th speech sample) also provide a channel for transporting signalling information associated with TFO, e.g. for the purpose of negotiating TFO operation between TFO partners. The remaining bits of the network coded data can still be used if necessary for fast fall-back to PCM by the reverse transcoder, for example if the TFO operation is suspended to allow the playing of a network announcement. For a short period whilst TFO operation is suspended, the quality of any mobile-to-mobile voice data will be degraded. The latest 3GPP specification for TFO (applicable to both GSM and UMTS) is TS 28.062 V5.4.0, see in particular chapter 4.2.
TFO operation must be negotiated by TFO partners (TRAUs and TCs). In the case of GSM, these would be located within the Base Station Subsystem (BSS) serving respective mobile terminals, or at Media Gateways located at core network interfaces. The TFO partners must establish that the call is mobile-to-mobile and that both mobile terminals are utilising the same codec. As there may be IPEs located in the network between the two TFO partners, the TFO negotiation must ensure that these are transparent to the TFO messages and frames. FIG. 1 illustrates schematically the nodes involved in TFO operation.
Considering TFO in more detail, a call is first initiated in the normal way, with the control plane inserting transcoders into the call path where necessary. Transcoding between the radio interface data and PCM starts immediately at the transcoders following call set-up. The Initial TFO protocol negotiation then starts between the TFO partners, with TFO messages being inserted in the LSBs of the PCM stream data bytes. First up to three TFO_FILL messages are sent to pre-synchronize potential IPEs into a transparent mode for TFO_Messages. Then the TFO protocol partners exchange information about their codec types and configurations by means of TFO_REQ and TFO_ACK messages. If the codec types and configurations are compatible the TFO connection will be established. TFO protocol then sends a TFO_TRANS message to switch the potential IPEs into transparent mode for TFO_Frames. TFO partners can now begin sending TFO frames with compressed speech as payload in the LSBs of the PCM signal. After any IPEs became transparent, a TFO partner will begin receiving TFO frames from the peer TFO partner and will stop its own encoder and start to use the parameters of the compressed speech that it receives in the TFO_Frames rather than the PCM coded data.
Both TFO_Messages and TFO_Frames contain synchronisation and signalling bits. In addition, they contain a so-called “Keep_Open_Indication”, which is a signal to the IPEs to stay in transparent mode. The Keep_Open_Indication consists of a “0” bit in the LSB of the PCM signal every 160 samples (20 ms).
The signalling flow involved in negotiating and maintaining TFO operation is illustrated in FIG. 2.
The setup of the TFO connection takes between 1 and 2 seconds, depending on the number of IPEs in the call path and certain other factors. This time is measured from the start of the first TFO_FILL message until the first TFO Frame is received. The setup time may be slightly different for the two directions.
When the call control layer requests the sending of a tone or an announcement on a link with an established TFO connection, this is achieved simply by replacing the PCM+TFO_Frame signal towards the distant TFO partner with a “pure” PCM signal of the tone or announcement, at the local TFO partner or at some IPE. The distant TFO partner will immediately detect the loss of TFO_Frame synchronisation and after a short while (about 40 ms) will start to use the received PCM signal and encode it into the compressed format that was used before the TFO break. The distant partner will stop sending TFO frames in the reverse direction, and the process will be repeated at the local partner, resulting in a breakdown in the TFO connection in both directions. After the tone/announcement sending is finished, the TFO protocol will re-establish the TFO connection.